The present invention relates to the measurement of the dimensions of an object by the emission of ultrasonic pulses and the reception of the echos returned by said object, whereby the time gap separating the reception of the different echos makes it possible to determine the distance separating the various echo-generating obstacles.
The invention has more particularly for its object the continuous measurement of the radial dimensions of cyclindrical tubes in order to check the values of the inside and outside radii of said tubes according to different azimuths.
The development of nuclear power stations has led to a considerable demand for tubes with very accurate dimensions, for example, tubes used in the heat exchangers of said nuclear reactors and the fuel sheathing tubes. This has led to research aiming at an improvement in the existing devices for the continuous measurement, for example by ultrasonics, of the dimensions of said tubes.
A known method for measuring the dimensions of a tube by ultrasonics comprises using two diametrically opposite transducers whose emitting faces are such that they transmit rays perpendicular to the tube axis and collect the rays reflected by the internal and external surfaces of the cylindrical tube, which makes it possible to measure the inside and outside radii. In order to observe the diameters, ovalization and various thicknesses of the tube according to different azimuths, it is possible to continuously displace the tube in front of transducers, whilst also rotating it about its axis so that a point on the tube surface has a helical movement and the different radii of the tube are measured for different aximuths. However, this type of device has disadvantages because the mechanical uncertainties and errors in the displacement of the tube, particularly due to the rotary movement thereof, lead to measuring errors. The same applies if the rotation of the tube is replaced by the rotation of the pair of transducers.
Another apparatus which avoids giving the tubes a rotary movement, whereby the continuous measurement of the tube dimensions is effected by a single translation of the cylindrical tube comprises arranging a series of transducers in a ring around the cylindrical tube axis, said transducers being grouped in pairs, whereby the two transducers of one pair of transducers face one another and simultaneously emit in order to measure the inside and outside diameters of the tube, whereby the different pairs of transducers arranged in the form of a ring are sequentially excited. However, this apparatus is relatively costly because it involves the use of a large number of transducers. The signal processing electronic system is complicated because it involves the sequential classification of the data collected by the transducers and in addition the measurement is only performed according to a discrete number of azimuths and in general according to four or six generating lines.